In the past, it is known to arrange an exhaust sensor in an exhaust passage of an internal combustion engine to detect a specific component in an exhaust gas (for example, see PLTs 1 to 3). This exhaust sensor is provided with an element body provided with an electrochemical cell, and a protective layer formed on the outer surface of the element body and comprised of a porous ceramic. The exhaust sensor is fastened to an exhaust pipe so as to be exposed to the exhaust gas. Part of the exhaust gas passes through the protective layer to flow into the element body. Further, the exhaust sensor is provided with a heater heating the element body so that the electrochemical cell becomes a predetermined operating temperature or more.
In this regard, when the temperature of the exhaust pipe is the dew point of water or less, the water vapor in the exhaust gas condenses and condensed water is produced. If there is condensed water in the exhaust passage, drops of condensed water strike the protective layer of the exhaust sensor together with the exhaust gas. If the protective layer does not have water repellency, the drops of water striking the protective layer penetrate the protective layer. If the temperature of the protective layer is high due to heating by the heater, the drops of water penetrating the protective layer evaporate inside the protective layer. As a result, thermal shock is given to the protective layer and element body and the element cracks.
Therefore, PLTs 1 and 2 describe to prevent cracking of the element of the exhaust sensor by utilizing the Leidenfrost phenomenon to impart water repellency to the protective layer of the exhaust sensor. The “Leidenfrost phenomenon” is the phenomenon wherein when drops of water strike a high temperature protective layer, a film of water vapor is formed between the protective layer and the drops of water whereby conduction of heat between the protective layer and drops of water is suppressed. If the Leidenfrost phenomenon occurs, the drops of water are repelled from the protective layer, so penetration of water to the protective layer is suppressed.
In the invention described in PLT 1, at the time of startup of the internal combustion engine, the Leidenfrost phenomenon is used so that the temperature of the sensor surface is held at an impact countering temperature higher than the operating temperature until the temperature around the exhaust sensor reaches the dew point so as to prevent cracking of the element.